Filling up on sun

In our mobile day-to-day life, the number of devices without rechargeable batteries is few and far between. Now, power storage devices are being eyed to give the energy transition a helping hand. TÜV Rheinland is making sure this meets with the intended success.

Lithium ion batteries are powering everything and everyone: They can be found in smartphones, E-bikes and electric cars. There are variants for every application, in numerous forms and with varying capacities. Now, these energy storage systems will help meet the biggest challenge of our time: the energy transition. As buffers, the bundled battery packs can buffer the naturally irregularly produced electricity from wind and solar plants until it is needed. In Germany, more than 35,000 electricity storage units have been connected to photovoltaic systems in private homes and commercial buildings since 2015. Every year, between 15,000 and 20,000 storage systems are added to this number. Existing installations are also being retrofitted – it’s now more attractive for operators to use as much of their own self-produced green electricity as possible instead of selling it and feeding it into the public grid. In the future, however, the power banks are slated to be part of a nationwide network of energy producers, storers and consumers who work together intelligently in the "smart grid". Networked, regenerative energy sources coupled with electricity storage devices could one day form the backbone of the decentralized power supply, meaning the continuously available power from conventional large-scale power plants would be rarely needed. Rechargeable energy would then not only provide individual households with green electricity, but whole cities.

Acid test for rechargables

In line with growing demand, companies are developing more and more powerful and – where possible – smaller, lighter and more economical rechargeable batteries. However, there are risks involved. "Lithium-ion batteries are being built with ever higher power densities. However, more energy in a small space inevitably also increases the potential for danger,” says Matthias Baumann, head of the TÜV Rheinland battery testing laboratory in Nuremberg. In test laboratories around the world, TÜV Rheinland is able to intensively test batteries of every design before they’re brought to market. The candidates are put through rigorous paces: week-long charging and discharging, deliberately applied overvoltage, extreme temperatures and physical stress. "There are two major problems with lithium ion batteries," says Romica Kiesewetter, test engineer on Matthias Baumann's team. "One is overcharging, which ends up depleting the battery. The other is extreme temperatures. In both cases, structures can form in the battery which trigger short circuits inside of it and, in extreme cases, cause a fire.” In addition, rechargeables in smartphones, toys and e-bikes are damaged by shocks, impact and falls. An example that resulted in headlines around the world was the batteries of the Samsung Galaxy Note 7 smartphone. Due to the design, the battery at times had too little space to expand when heating up inside the phone. The result: Numerous batteries suddenly caught fire, and the manufacturer had to recall roughly three million devices, and finally took the model off the market altogether. During the subsequent effort to establish the cause, TÜV Rheinland was able to help Samsung to eliminate potential quality problems during transport and installation of the batteries. Rechargeable batteries that successfully pass the test cycle at TÜV Rheinland have no such problems when used correctly.

Power from the shoe box

Rechargeable batteries for storing solar power are now regularly tested by TÜV Rheinland. The giant batteries for the basement aren’t only different in size from the smartphone batteries shaped like a bank card. They’re much more robust and built for a decade of continuous operation. Lithium-iron phosphate cells are usually used. They can be charged quickly, but they also lose their energy quickly. Several cells are joined together in shoe-box-sized modules, or battery packs. Depending on needs, dozens or hundreds of these modules are installed in refrigerator-sized storage enclosures, or fill entire containers. Unlike batteries in smartphones, due to the way they’re constructed the cells usually cannot overheat. An electronic control system prevents incorrect charging and discharging – a common cause of battery death in laptops, drills and electric toothbrushes. During gentle operation, all the cells of a storage device are always charged evenly and only 80% discharged, which enables them to attain a large degree of their original performance even after some 5,000 charging cycles. The cells are designed to last over 20 years, which corresponds to the economic life of conventional energy systems such as gas and oil heating systems. If they subsequently wear out one day, this poses no problem: The cells can be recycled into new energy storage devices, which is wholly in line with the thrust of the energy transition.